1. Field of the Invention
The present invention is directed to a method for imaging an area of investigation by means of a source half-tone picture of the area of investigation, the source half-tone picture being composed of individual picture elements, wherein the picture elements are allocated source gray scale values corresponding to the morphology of the area Of investigation, and employing a framing mask operation which produces modified gray scale values from the source half-tone pictures as a function of a preassigned brightness value and a preassigned contrast value, which are fed to a display device.
2. Description of the Prior Art
Half-tone pictures of an area of investigation are used and evaluated in various fields of medicine and technology. From experience it is known that retinal rods control the monochromatic sensing of light and dark, while retinal cones sense color. The best half-tone differentiation of gray scale values, termed scotopic or rod sight, takes place in a darkened room. Even under optimal conditions, however, only 15 to 40 gray levels can be distinguished. Half-tone pictures, in particular those used in medicine, are typically analyzed in a darkened room.
The adaptation of the human eye to darkness is strenuous, approximately 30 minutes being required before the eye has completely adapted to a dark environment. Interim studies under brightness adaptation conditions either resulted in time delays,.until the eye has adapted completely to a new darkness, or to a reduced half-tone sensing, during which the eye had not completely adapted to darkness.
A brightness adaptation of the half-tone picture to the ambient brightness by modifying the monitor amplification or by amplifying the image-producing system can, however, lead to glare and subthreshold artifacts. A manual tracking of the image contrast cannot prevent image artifacts with absolute certainty. The adaptation of brightness and contrast to ambient conditions is costly and requires considerable experience on the part of the user,
A method for improving contrast in the display of an image, using a framing mask operation, is known from the abstract titled, "Artifact-Resistant Gray Scale Windows in Clinical Ultrasound of the Liver" by Bleck et al, published in Book of Abstracts, 19th International Symposium on Acoustical Imaging, Apr. 3-5, 1991, University of the Ruhr, Bochum, Germany. As described therein, various framing mask operations were investigated to determine whether they offer a contrast improvement. This type of solid framing mask operation provides the opportunity to present certain types of tissue more clearly in medical displays. Imaging free from artifacts under any ambient conditions, however, cannot be achieved solely by preassigning a specific mask operation.
Also prone to artifacts is a histogram-oriented framing mask operation, as described by C. A. Harlow et al in Chapter 3.3.2 under the heading Preprocessing" in "Digital Picture Analysis", edited by A. Rosenfeld, Springer-Verlag, Berlin, Heidelberg, New York--1976, pp. 80-83. The histogram comparison cited in this work leads to artifacts in bright and dark image areas.